Conservation of aquatic plants in artificial watercourses: are main drains a substitute for vulnerable navigation canals?

1. The vegetation of disused and occasionally used navigation canals with acknowledged botanical conservation value was compared with that of major land-drainage channels with potential conservation interest. 2. Aquatic plant taxa and their abundance were recorded in 500-m lengths of watercourses in East Yorkshire, north-east England. 3. There was substantial overlap between canals and drains in the aquatic plant species recorded (42 out of 66 taxa were found in both canals and drains) and ordination analysis (DECORANA) showed that there were not two distinct vegetation types: one for canals and one for drains. 4. The aquatic plants recorded in both canals and drains probably largely originated from an eighteenth- and nineteenth-century pre-drainage marshland flora that has since been lost. 5. The conservation value of the aquatic vegetation at canal sites (assessed as diversity, rarity and naturalness) was not significantly greater than at the drain sites. 6. The canals are potentially threatened by restoration of navigation or increase in boat usage; a simplistic interpretation of the results is that potential losses in the conservation value of canals would be substituted by the flora of the drains. 7. A more measured interpretation, however, is that the canals and drains should be regarded as parallel conservation resources. This is because some aquatic plant taxa with limited distribution at local, regional or national level were found only in canals while others were found only in drains. 8. The results emphasize that evaluation of the botanical conservation value of artificial watercourses should include drains as well as canals; disused and lightly used navigation canals are widely recognized as of high conservation value whereas drains, being utilitarian landscape features, are more likely to be overlooked. Copyright (c) 2007 John Wiley & Sons, Ltd.